Photometers are generally known for measuring the light permeability of a reagent liquid having a predetermined proportion of a test substance mixed with the reagent liquid. The many uses for such a photometer include a determination of the proportion of hemoglobin in human blood. Prior art photometers for such purposes have been laboratory instruments that were not readily portable and were relatively slow and cumbersome to use. Moreover, the output value from prior art instruments required a calculation to adapt the valve to the particular test, and no direct reading of the output was available.
The invention involves recognition of the need for a portable photometer that can be handheld and operated for quickly and reliably making photometric measurements outside of laboratories. For example, one need for such a portable photometer is to allow doctors to measure the proportion of hemoglobin in human blood quickly and reliably at the scene of an accident or under emergency conditions. The invention aims at a conveniently portable photometer that is simple and easy to operate, accurate and reliable in making measurements, and providing a directly readable display of the test result value from any measurement.